According to conventional practice, the process of transforming a tract of land into a graded surface involves several tasks, typically beginning with the task of surveying the land in order to create a contour map or other graphical representation of the pre-existing state of the land. Surveying involves the delineation of the form, extent, and position of the tract of land based on linear and angular measurements of the land. Conventional surveying is at least a two person job, with one person operating a measuring instrument from a generally stationary position and the other person transporting and positioning a grade rod or other reference to be sighted by the measuring instrument.
The measuring instrument, such as a transit, theodolite, distance meter, or total station, is positioned at a known distance and angle from a reference, or bench position. The grade rod is sequentially positioned at one or more locations, and at each such location, the distance and angle of the grade rod with respect to the position of the measuring instrument is determined and recorded. Distances may be measured manually with a steel tape or chain, or may be measured optically by the measuring instrument utilizing various means such as a retroreflector on the grade rod. Angles are typically measured in both horizontal and vertical planes, with an azimuth angle defined as the horizontal angle measured clockwise from north, and a zenith angle defined as the vertical angle measured downward from the vertical.
From the distance and angle information obtained in the survey, and through application of the principles of geometry and trigonometry, the surface of the tract of land can be characterized and presented in graphical form. The position or location of any point on the tract of land can be represented in a variety of three-dimensional coordinate systems such as X, Y, Z, or R, θ, Z, where X, Y, Z denotes a Cartesian coordinate system in which the X-Y plane is horizontal and the Z-axis is vertical, and where R, θ, Z denotes a cylindrical coordinate system in which the R-θ plane is horizontal and the Z axis is vertical. The X, Y or R, θ coordinates are measured in a horizontal plane with respect to some bench mark position, while the Z coordinate is the elevation measured with respect to some horizontal reference plane, such as mean sea level.
After the tract of land has been surveyed, a site plan can be drawn up to define what the contours and elevations of the land should be after grading. In accordance with conventional practices, the site is then marked with stakes in order to guide the operators of earth-moving equipment while they grade the land into conformity with the site plan. The process of marking involves first defining on the site plan the coordinates of various key locations to be marked, and then placing stakes on the land at those locations. The task of marking the land can utilize the same surveying apparatus described above. The grade rod is roughly positioned near a location to be marked, and its position is determined by the measuring instrument. If the grade rod is not exactly positioned at the location to be marked, the position is noted and the grade rod is repositioned and remeasured until the measuring instrument verifies that the grade rod is positioned at the location to be marked. A stake or other marker is then driven into the ground at that point. Like surveying, the conventional process of marking a tract of land is also a task that requires at least two trained people.
In order to designate the desired elevation at the marked locations, the stakes are typically marked with indications of the depth of fill or cut needed to create the desired graded surface at those locations. Such fill or cut information can be determined according to the elevational differences between the existing ground site and the site plan.
After the tract of land has been marked, earth-moving equipment can be used for grading the site. The operators of the earth-moving equipment are guided by the marker stakes in determining where to cut and where to fill. Care must be exercised to avoid damaging the stakes during the grading operation. The site may need to be re-surveyed during or after completion of the grading to verify the accuracy of the graded surface. With the necessary tasks of surveying, marking, and resurveying, the convention practice of transforming a tract of land into a graded surface is unavoidably labor intensive, even apart from the actual grading operations.
To automate grading of the surveyed land, automatic control systems for earth-moving equipment have been developed to control the elevation of the grading implement. As best viewed in FIGS. 1 and 2, these first generation automatic laser grade control systems 10 typically include a laser transmitter 11, a laser receiver 12, a control box 13 and a hydraulic valve 14. The valve 14 is then operably coupled to the hydraulic rams 15 to automatically control the raising or lowering of the blade 16 of the earth-grading apparatus 17.
Generally, the laser transmitter 11 includes a rotating laser beacon 18 which sweeps out a plane 19 of pulses of light 20 parallel to the desired graded surface. In ordinary operation, the pulses of light 20 from the rotating beacon 18 strike light sensitive cells 21 in the receiver 12 mounted on the machine blade 16, typically through rod 22 or mast. As better illustrated in FIG. 3, the receiver cells 21 are arranged in a vertical array with the vertical displacement from the array's center corresponding to the amount of grade correction required, resulting in coarse, fine or on-grade correction signals. Correction signals from the receiver 12 are processed through the control box 13 to an electrically actuated hydraulic valve 14 which drives the hydraulic rams 15 to raise or lower the blade.
While this automatic laser grade control system is capable of precise automatic control of the blade elevation through control of the hydraulic valve, this control is essentially one dimensional. That is, these systems, provide only planar control of the blade that is otherwise independent of the blade's location on the site, and generally can be satisfactorily applied only to those portions of the site plan which are large planar surfaces. Typical of these automatic laser control systems is the System IV™ with laser receiver manufactured by Topcon Laser Systems, Inc., of Pleasanton Calif., the concept of which was disclosed in part in U.S. Pat. No. 3,494,426, herein incorporated by reference in its entirety.
More recently, three dimensional grading guidance systems 30 and three dimensional grade control systems have been introduced to overcome the limitation of planar dimensional systems.
As shown in FIG. 4, a three dimensional grading guidance system 30 generally includes an optical or GPS real time three dimensional positioning system 31 with the measure point in the machine's blade 16. A three dimensional computer model 32 of the grading plan is also required which is coupled to a processing and display device 33 to calculate the difference in the elevation measured by the positioning system 31 from that of the model at the same horizontal coordinates. Control of the blade is performed manually based upon the grading guidance information provided to the machine operator. An example of a GPS real time three dimensional grading guidance system is the SiteVision™ system manufactured by Trimble Navigation Limited of Sunnyvale, Calif.
Automatic three dimensional grade control systems have also been developed which are capable of precise automatic control of the blade elevation through control of the hydraulic valve. An early example of an automatic three dimensional grade control system is disclosed in part in U.S. Pat. No. 4,820,041. Current commercial examples of these automatic three dimensional grade control systems are the 3DMC system manufactured by Topcon Laser Systems, Inc. of Pleasanton, Calif., and the Bladepro3D system manufactured by Trimble Navigation Limited of Sunnyvale, Calif.
For many precise grading applications, automatic three dimensional grade control systems are superior to three dimensional grade guidance systems, but they are inherently more costly because they required many additional control components. The additional control components are already present in existing conventional automatic laser grading systems. Accordingly, it would be desirable to adapt an existing conventional laser grading system to a three dimensional data base and a three dimensional positioning system such as a robotic total station or real time kinematics GPS system for a cost effective solution to automatically control the blade of the earth-moving apparatus.